weirdSnake picoCTF 2024 Solution

Description

1. Step 1Recover the key
   Disassemble the bytecode and scan for a sequence of LOAD_CONST ops that push single characters onto the stack, ending in BUILD_STRING. That sequence reconstructs key_str (in this challenge it builds out to t_Jo3). Convert it to a list of integers for the XOR step:
   python

   python3 -m dis snake | less

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   bash

   key_list = [ord(c) for c in 't_Jo3']

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   Python source files (.py) are compiled to bytecode (.pyc) by the CPython interpreter before execution. Bytecode is a lower-level, platform-independent instruction set for the Python Virtual Machine: not machine code, but not source code either. It sits in the middle: harder to read than Python, but much easier to recover than compiled C or C++.

   Tools like uncompyle6, decompile3, or pycdc can reconstruct Python source from bytecode with high fidelity. Python's built-in dis module disassembles bytecode into human-readable opcode mnemonics; useful when decompilers fail on unusual or obfuscated bytecode.

   • python3 -m dis snake disassembles the bytecode file.
   • uncompyle6 snake attempts full source reconstruction (install with pip install uncompyle6).
   • Strings built character-by-character show up as repeated LOAD_CONST 't', LOAD_CONST '_', ... followed by a BUILD_STRING N op that joins them.
2. Step 2Recreate the input list
   The bytecode stores the ciphertext integers in a list (input_list). Copy those numbers into your script.
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   In Python bytecode, list literals are stored as a series of LOAD_CONST instructions (one per element) followed by a BUILD_LIST opcode. The disassembly makes these values directly visible - no decryption needed to extract the ciphertext list. This is a fundamental asymmetry in obfuscation via compilation: the data the program operates on must be present at runtime, so it is always recoverable from the bytecode.

   ord(c) converts a character to its ASCII integer value (e.g., ord('A') == 65). The key is stored as a string but used as a list of integers during XOR operations, so converting it with [ord(c) for c in key_str] produces the correct numeric representation.

   Recognizing that a program stores integers in a list and XORs them against a key is a pattern found constantly in CTF reversing challenges and in real malware - it is one of the simplest ways to obfuscate strings like API keys, C2 addresses, or flags without using a proper cipher.

3. Step 3XOR decrypt
   XOR each ciphertext byte with the cycling key. Watch out: zip(a, b) stops at the shorter sequence, so if you pass the raw key_list and it is shorter than input_list you silently lose the tail of the message. Use itertools.cycle to repeat the key. The Python for CTF guide covers more idioms like this.
   python

   from itertools import cycle
   result = ''.join(chr(a ^ b) for a, b in zip(input_list, cycle(key_list)))
   print(result)

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   XOR (exclusive OR) is the simplest symmetric cipher. XOR has a beautiful property: applying the same key twice returns the original value (m XOR k XOR k = m). This makes XOR decryption identical to encryption: just XOR the ciphertext with the same key and you recover the plaintext.

   When the key is shorter than the message, it is typically repeated cyclically; this is called a repeating-key XOR or Vigenere-like XOR. zip in Python pairs elements from two sequences and stops at the shorter one, which is the gotcha called out above. itertools.cycle(key_list) creates an infinitely repeating keystream so the zip stops at the ciphertext length instead.

   Repeating-key XOR was used in real cryptography before modern ciphers (the Vigenere cipher is a letter-based variant). It is broken by known-plaintext attacks (if you know any bytes of the plaintext, you immediately recover the corresponding key bytes) and by frequency analysis when the key is short relative to the message. AES in CTR mode is essentially XOR with a secure, non-repeating keystream; the security comes entirely from the quality of the keystream, not the XOR operation itself.

Alternate Solution

Related guides

Flag

The decrypted string from the XOR routine is the final flag.